1.1 Field of the Invention
The present invention is a methodology for optimizing performance of a standoff optical spectroscopy system using measurements of optical inelastic scattering from an intermediate medium between the sensor and target and/or utilizing optical inelastic scattering from the target.
1.2 Related Art
Optical range-finding or LIDAR (light detection and ranging) is a well-known method for measuring distance. An LIDAR range-finder works by emitting pulses of light (typically infrared but visible or ultraviolet are also possible) towards the target and detecting reflected light from the target. The round-trip travel time of the light to the target and back can be measured and the distance calculated from that time. For instance, Endo (U.S. Pat. No. 4,634,272 A) and Schwartz (U.S. Pat. No. 4,518,256 A) teach methods of using an optical transmitter and array of optical sensors to detect multiple objects and measure their distance and direction. One weakness of LIDAR range-finding and object detection is that it requires the target to be retro-reflective, i.e. some portion of the light must elastically scatter (reflect) off the target back to the rangefinder. Targets with anti-reflective coatings, absorptive coatings, or even transparent targets can degrade the detection performance by reducing the elastic scattered signal. Another weakness of LIDAR is that measuring target range through an intermediate medium that is partially reflective, such as smoke or fog, significantly complicates the measurement, because these materials retro-reflect a portion of the light back to the receiver, generating signals at times earlier than the signal from the target. Another weakness of LIDAR is that it can be jammed by a target or nearby object emitting pulsed light at the same wavelength.
Standoff optical spectroscopy (SOS), such as standoff spontaneous Raman spectroscopy or standoff diffuse reflectance spectroscopy, is a useful technique for analyzing material composition in situations where contact with the analyte is either not desirable or not feasible. SOS is typically performed by projecting light from an ultraviolet, visible, or infrared source, such as a laser, onto a target of interest which is some distance away, and collecting the scattered light onto a detection system which can measure the scattered light intensity at one or more wavelengths or ranges of wavelengths.
One common detection system for standoff optical spectroscopy utilizes spontaneous Raman scattering. A common standoff Raman spectroscopy system comprises at least one monochromatic optical source, typically a laser, a transmit subsystem to direct light from the source towards a target, a receive subsystem comprising optical elements to gather/collect light from the target, optical elements to filter out undesired wavelengths, a device for selecting a wavelength to impinge on a detector or a device to disperse light of a range of wavelengths onto elements of a detector, a detector for measuring the intensity of the selected wavelength(s) of light, a control subsystem for coordinating the actions of the light source, detector and wavelength-selective elements, an analysis subsystem which records and processes signals from the detector and any ancillary sensors (such as sensor and air temperature, atmospheric pressure, laser power, or other factors which may influence the measured spectrum), and other mechanical, electrical, computer, communication, and user interface subsystems as needed. The foregoing combination of components and their operation to interrogate targets at stand-off distances with excitation energy and detect presence of chemical species of interest by evaluation of inelastic scattering from the target with Raman spectroscopy, is well-known to those of skill in this technical field. A few examples, commonly-owned by the owner of the present invention, are: U.S. Pat. No. 8,125,627 to inventors Dottery, et al.; U.S. Pat. No. 8,724,097 to inventors Pohl et al.; and U.S. Pat. No. 9,157,801 to inventors Dottery, et al.; each of which is incorporated by reference herein. Other configurations are well known. The transmit subsystem and the receive subsystem may be integrated into a single assembly (in which case the entire assembly is typically referred to as the detection system) or may be separated.
Since the relative intensity of Raman-scattered light at various wavelengths depends on the chemical makeup of the material scattering the light, chemical analysis of the target is accomplished by analyzing the wavelength spectrum of the returned light. For standoff Raman spectroscopy, it is often desirable to separate the Raman spectrum from light scattered by the target to be analyzed from the Raman spectrum generated by interactions with other objects or materials (herein called “background”). Common causes of background include Raman scattering from an intermediate medium such as air between the detection system and the target, intermediate material such as dust or objects that partially occlude the target, material beyond the target if the target does not completely occlude the emitted light, and material not in the intended light path which generates signal due to multiple scattering events.
A method for separating the desired spectrum from the background spectrum in standoff Raman spectroscopy as taught by Dottery U.S. Pat. No. 9,157,801 is to utilize a pulsed emission source with a short duration (typically less than 100 nanoseconds) and a detection system which can separate, measure, or select signals based on received time. This is closely related to the concept of light detection and ranging (LIDAR) in which ranges are calculated using the delay between emission and detection time, although LIDAR systems typically utilize detection of reflected light rather than inelastic scattered light. Reflected light (elastically scattered light) is light scattered without a meaningful change in wavelength. As is well-known in the art and expressed in published patent application US 08/0198365 A1 to inventors Treado, et al. which is incorporated by reference herein, Raman scattering spectra is “inherently richer” in molecular-specific “fingerprint signatures” and therefore can be a valuable tool for molecular identification of materials. This is particularly true of many explosive materials, because they have strong, unique Raman spectra that are essentially “fingerprints” of the vibrational spectrum of such molecules.
Such non-destructive, contactless, stand-off interrogation of materials has highly significant potential. One context is remote detection of explosives or otherwise hazardous materials. It can literally be a life-saving tool. However, the challenges for accurate, timely, and practical techniques are many. One is spatial. Sufficient stand-off interrogation distances (sometimes tens if not hundreds of meters) make it difficult to ensure and know if the excitation energy is optimally interrogating the target and if the light energy collected thereafter is optimally from the target and not from other materials (including the molecules that make up ambient air between detector and target). Another is temporal. Can the technique obtain and process sufficient information in a short-enough time to be practical? Although interrogation and return scattering are essentially at the speed of light, the ability to coordinate, collect and process complex content is in the microsecond, nanosecond, and even shorter time scales. This adds a level of complexity and unpredictability. Another is signal-to-noise ratio. Raman content is notoriously weak relative to other content in light collected from the target, so it is difficult to extract. Another is accuracy and precision. The difficulties in translating spectra from scattering from an unknown material into identification of molecules is well-known.
The quality with which the desired Raman signal can be separated from the background Raman signals is largely determined by a combination of factors including (1) the duration of the emission pulse, (2) the accuracy and precision with which the received signal time can measured or selected, (3) the accuracy and precision of the known distance to the desired target, and (4) the accurate quantification of any timing delays within the system.
The duration of the emission pulse affects the separation of background signals from target signals because the received spectrum at any given time is a summation of emissions generated over a range of distances which is directly proportional to the pulse duration. For a pulse duration of ΔT, the received signal at time T after the beginning of the emission pulse is a summation of signals generated at ranges of
      c    2    ⁢      (          T      -              Δ        ⁢                                  ⁢        T              )    ⁢          ⁢  to  ⁢          ⁢      c    2    ⁢  Twhere c is the speed of light. Therefore, a short pulse duration is desirable, since this will mix less signal from background materials in front of or behind the target with signals from the target. Timing accuracy and precision controls a similar mixing of background and signal, because to ensure all the signal from the target is collected, the signal collection duration must be larger than any timing errors.
In one standoff Raman implementation, the receive system incorporates an electro-optical gate component or capability, such as an intensifier tube, photomultiplier tube, Kerr gate, or other. These types of gates are well-known in to those skilled in the art, including how to implement them. Signal light is allowed through the gate for a set period of time (starting at a time Tdelay after pulsed emission from the source width until a later time Tdelay+Tgate), with Tdelay typically selected based on target distance as measured or estimated using some means such as a rangefinder. In another implementation, the receive system incorporates a detector and signal processing system which are able to accurately measure signal intensities and variations at rates sufficient to select the signal at a particular time of interest. In this case, the receive system typically requires a bandwidth in excess of 10 MHz (and sometimes in excess of 100 MHz). Again, it is common practice to use a measured or estimated target distance to predetermine which portions of the processed signal are largely due to desired signal.
A few examples, all incorporated by reference herein, of various laser/detector/spectrometer systems for evaluating inelastic scattering include: U.S. Pat. No. 8,072,595 to inventors Bastiaans et al. (discussing use of a programmable delay generator triggered by firing of a laser including for elastic or inelastic scattering optical time domain detection; U.S. Pat. No. 2008/0198365 to inventors Treado, et al. (discussing why and how inelastic scattering and Raman spectra can be used to identify molecular species); and US 2010/0309464 to Treado et al. (discussing Raman chemical imaging using pulsed laser excitation and time-gated detection).
Regardless of the timing selection method used, it is common practice to use a separate, dedicated device, subsystem, or method to pre-determine the target range. The timings can then be calibrated based on the speed of light as well as any delays that are inherent to the system. Alternately, for some applications the distance to target can be fixed and all timings can be calibrated to that specific range.
In addition to controlling timing parameters of the system, an accurate distance measurement to the target is often desirable in order to focus the optical transmit and/or receive subsystems to optimize detection performance at that range, to optimize other controllable parameters such as data collection time, or to monitor or control other aspects of the system such as range interlocks.
There are several issues with the existing methodologies for determining range. A fixed range is generally only useful in a controlled laboratory setting as controlling the range precisely is generally not feasible in commercial settings. Using a separate range finder device or method also has several drawbacks. Range finders use a variety of methods to determine the range. Regardless of the methodology, the failure modes of the range finder are likely not the same failure modes as the standoff optical spectroscopy system, i.e. the range finder can fail in situations where, had the range been accurately determined, the spectroscopy would have succeeded in analyzing the target. Such situations may be different than situations in which the spectroscopy could not have succeeded at all. For instance, a very highly retro-reflective object can cause a LIDAR range finder to receive a signal much stronger than expected, saturating the receive electronics, but will not significantly impact the spectroscopy which is only weakly affected by the reflectivity.
Another issue is that by having a secondary range finder device the range finder either has to be bore sighted to the primary laser of the spectroscopy system, which adds additional complexity to the overall system, or the range finder will have a differing line of sight than the primary laser, adding uncertainty to the accuracy of the measurement. A secondary line of sight introduces the possibility that the range finder will strike a target that is not the actual target of interest and could potentially give the incorrect range to the target reducing the apparent reliability of the system.
Finally, a range finder will produce a range to the target that then must be converted into the correct gate timings (Tdelay, Tgate) for the system. This introduces another possible source of error in that the calibration may either be done incorrectly or that there are delays inherent to the system that are not constant which again reduces the overall system accuracy or increases the system complexity.
Therefore, a need exists for a methodology of determining the gate timings required without using a supplementary device such as a range finder. This will reduce the complexity and increase the accuracy and reliability of such detection systems. As will be appreciated by the foregoing, the beneficial potential for such systems and techniques is tremendous. Yet the competing factors to make them accurate and practical make solutions elusive and unpredictable. This is evidenced by the many different attempts and approaches in the state the art. Working at relatively large distances, but at nano- and even pico-second time domains with substantial signal-to-noise, are imposing challenges.